1. Field of the Invention
In general, the present invention relates to an exposure apparatus. More particularly, the present invention relates to an exposure apparatus used to expose a material to be processed, such as a monocrystalline substrate for a semiconductor wafer or a glass substrate for a liquid crystal display (LCD), a correcting device that corrects deformation of a mask or a reticle (hereafter, these terms are used interchangeably in the application) used in the exposure apparatus, a device production method using the material to be processed, and a device that is produced from the material to be processed. The present invention is, for example, suitable for application to an exposure apparatus which exposes a monocrystalline substrate for a semiconductor wafer by the step-and-scan projection method, the scan projection method, or the step-and-repeat projection method in a photolithography process.
Here, the step-and-scan projection method is a projection exposure method in which a wafer is continuously scanned in synchronism with a scanning movement of a mask or a reticle in order to project a pattern of the mask onto the wafer by exposure, after which, after completion of an exposure of one shot, the wafer is moved stepwise in order to move the next shot to an exposure area. The scan projection method is a projection exposure method in which a portion of the mask pattern is projected onto the wafer by a projection optical system and the mask and a material to be processed are scanned in synchronism with each other with respect to the projection optical system in order to project the whole mask pattern onto the wafer by exposure. The step-and-repeat projection method is a projection exposure method in which the wafer is moved stepwise with each full exposure of a shot of the wafer in order to move the next shot to the exposure area.
2. Description of the Related Art
In recent years, the demand for smaller and thinner electronic devices has caused an increasing demand for finer semiconductor devices installed in the electronic devices. For example, it is expected that design rules of a mask pattern will become increasingly smaller in the future as a result of an attempt to realize a line and space (L & S) of 130 nm in a mass production line. L & S refers to an image projected onto a wafer during exposure with the widths of the lines and spaces being equal, so that it is a measure of exposure resolution. In the exposure, resolution, overlay accuracy, and throughput are three important parameters. Resolution is defined as the smallest dimension that can be precisely transferred. Overlay accuracy is defined as the accuracy with which several patterns are overlaid on a material to be processed. Throughput is the number of materials that are processed per unit time.
There are basically two types of exposure methods, a 1× magnification transfer method and a projection method. The 1× magnification transfer method includes a method in which a mask and a material to be processed are brought into contact with each other and a method in which they are separated slightly. However, in the former method, although a high resolution can be obtained, the mask gets damaged and the material to be processed gets scratched or defective due to dust or pieces of silicon being pressed into the mask. In the latter method, the problem that exists in the former method is initially solved, but, when the separation between the mask and the material to be processed becomes smaller than the maximum size of dust particles, damage to the mask similarly occurs.
To overcome the problem that the mask and the material to be processed become damaged, a projection method in which the mask and the material to be processed are further separated has been proposed. Of the different types of projection methods, the projection method that uses a scanning projection exposure apparatus is in dominant use in recent years in order to improve resolution and to increase the size of an exposure area. In this projection method, the mask is exposed a portion at a time, and the mask and the wafer are caused to be in synchronism with each other. By scanning the wafer either continuously or intermittently, the entire mask pattern is projected onto the wafer by exposure.
In general, a projection exposure apparatus comprises an illumination optical system that illuminates a mask and a projection optical system, disposed between the mask and a material to be processed, which projects a circuit pattern of the mask that has been illuminated onto the material to be processed. In the illumination optical system, in order to obtain a uniform illumination area, light beams from a light source are made to enter a light integrator comprising, for example, fly's eye lenses that are provided using a plurality of rod lenses. With a light-exiting surface of the light integrator being used as a secondary light source surface, these light beams that have entered the light integrator are used to subject a mask surface to Koehler illumination through a condenser lens.
However, when the optical axis substantially coincides with the direction of gravitational force, the center portion of the mask is flexed by an amount on the order of a few microns in the direction of the gravitational force due to its own weight, resulting in a problem that overlay accuracy is reduced during the exposure. More specifically, the following problems arise: (1) Distortion of a projected image of the pattern changes as a result of distortion of the mask pattern, and (2) focal depth, which is the focal range that allows a certain image-formation performance to be maintained, is reduced by curvature of field. In particular, it is expected that due to the recent demand for finer patterns, even a slight variation in the pattern must be increasingly taken into account in the future.
To overcome such problems, Japanese Patent Laid-Open No. 10-214780 proposes, in a first embodiment, to enclose a mask in order to apply static pressure to a hermetically sealed space through a pressure control device. However, when the mask is enclosed, heat produced by exposure causes the mask to be distorted, so that this method is not a preferable method. In addition, the same document proposes, in a second embodiment, to correct the distortion of the mask through a piezoelectric device disposed around the mask. However, the use of the piezoelectric device around the mask is not necessarily effective in removing flexure of the center portion of the mask caused by its own weight.
Japanese Patent Laid-Open No. 6-176408 proposes to supply gas having a predetermined pressure to a mask from a direction opposite to the direction in which the mask flexes. However, it is difficult to uniformly apply pressure to the mask. In addition, it is difficult to dispose gas blowing means while maintaining an exposure optical system.